The present invention relates generally to protective relays for protecting an electrical generator of the type characterized by output performance limit curves, and more particularly, to a protective relay having an operational boundary matched substantially to the output performance limit characteristics of the generator to enhance the protection thereof.
Typical generator output performance limiting curves are shown in the graph of FIG. 1 which depicts the real or effective power component of the generator on the abscissa and the reactive power component of the generator on the ordinate. Curves 11 and 12 denote delay and lead reactive power component limit curves of the generator and curve 13 denotes an effective or real power component limit curve of the generator. The hatched zone in FIG. 1 illustrates a power output zone in which the generator may continuously operate.
FIG. 2 is a graph illustrating the output performance limit curves of the generator in the form of an impedance diagram converted from the curves 11, 12 and 13 described in connection with the graph of FIG. 1. For example, curve 12a denotes an impedance conversion of curve 12 and represents the lead reactive component of the generator terminal, and curve 13a denotes an impedance conversion of curve 13 and represents the reactive component of the generator terminal associated therewith. In FIG. 2, the hatched areas outside of the curves 12a and 13a represent the permitted generator output performance areas. In addition, the hatched area 2 indicates the operating area of a typical field failure relay having a generator protecting function which utilizes the fact that the impedance calculated from the terminal voltage and current of the generator moves toward the center of a circle when the field of the generator is obviated. The area between the hatched circle 2 and the output limit performance curves 12a and 13a represents an area which exceeds the performance limits of the generator. Accordingly, if the generator is operating in an area like that shown by the hatched circle 2a, for example, it cannot be protected by the field failure relay having a protection zone exhibited by the hatched area 2.
From the above remarks, it is readily apparent that the conventional protection scheme for a generator has a disadvantage in that the field failure relay protective zone is not matched to the output performance limit curves of the generator, like 12a and 13a, for example, leaving an unprotected operational area such as that shown by the hatched area 2a which exceeds the output performance limits of the generator. The present invention proposes to eliminate substantially the aforementioned disadvantage and to provide a generator protective relay which has an operating boundary which is matched substantially to the output performance limits of the generator being protected.